In the drilling of wells for production of oil, gas, and the like, it has been known to use down hole drilling assemblies having fluid motors to rotate the drill bit rather than rotating the entire drill string. In such drilling operations, a drill string comprising lengths of pipe, extends down the well bore having the drilling assembly secured at its lower end.
The drilling assembly has a casing, usually screwed to the bottom end of the pipe string, which includes one of two types of fluid motors. One type is a multi-stage progressive-cavity positive-displacement motor and the other type of motor is a turbine. The motor has a rotor coupled to a shaft. The shaft, in turn, mounts the drill bit. The shaft is contained within a housing connected to the casing by suitable means.
Drilling fluid, in the form of a special "mud", is pumped down the drill string to the drilling assembly to drive the fluid motor. The mud is admitted to the casing and the motor which, due to the energy of the pumped mud, rotates the rotor shaft and drill bit. The mud leaving the motor is directed through the shaft to the bit and the well bore to cool the bit and remove rock fragments from the well.
To journal the shaft for rotation, the housing includes a bearing assembly. The bearing assembly must be capable of withstanding and transmitting various thrust (axial) loads as well as radial loads between the shaft to the housing. For example, to drill the well, weight is applied to the drill string and drilling assembly to force the drill bit against the rock formation. This is referred to as the "on-bottom condition", that is, when the rock bit is on the bottom of the hole. When at the on-bottom condition, the shaft imposes an up-hole thrust loading on the bearing assembly which must effectively be transferred to the housing. These on-bottom thrust loads can be many tens of thousands of pounds and must be accommodated by the bearing assembly.
When the rock bit is off the bottom of the hole, and the motor is operated, the mud pumped down the well string applies a down-hole thrust on the rotor of the motor. This down-hole thrust is transmitted to the shaft resulting in a down-hole thrust load upon the bearing assembly. The bearing assembly must transmit these off-bottom, down-hole thrust loads between the shaft and housing. Off-bottom thrust loads can be as large or larger than the on-bottom thrust loads. While total operating time for the off-bottom loading may be less than the operating time for the on-bottom loading, such time intervals can be substantial.
To drill a hole, ordinarily mud flow is commenced with the rock bit off bottom. The drill string is then lowered to bring the drill bit to an on-bottom condition in engagement with the rock formation. Drilling continues for a length of one joint of pipe (e.g. 60 or 90 feet). The drill bit is then lifted off bottom and mud flow is terminated while a new joint of pipe is assembled onto the drill string. The cycle is then repeated to complete a desired depth hole. Thus the bearing assembly is continually subjected to reversals between up-hole and down-hole thrust loading.
In addition to thrust loading, the bearing assembly also transmits substantial radial loads between the housing and shaft, particularly when the drilling assembly is turned to change the direction of the hole. Changing the direction of the hole is often referred to as deviation. During deviation, the shaft radially loads the bearing assembly by virtue of gravity (i.e., the weight of the shaft) and by bending forces on the shaft as deviation is made. Loading by bending can be reduced by shortening the drill assembly which typically can be more than 42 inches long for a six and one-half inch diameter assembly. Reduction of bending is important to reduce bending fatique on the shaft and radial loading upon the bearing assembly.
Previous bearing assemblies for down hole drill assemblies have required radial bearings at the upper and lower ends with intermediate thrust bearings to journal the shaft and transmit radial and thrust loads repsectively to the housing. For example, in Ioanesian, et al, U.S. Pat. No. 3,944,303, issued Mar. 16, 1976, a typical drilling assembly is shown. To accommodate the thrust loads, multiple sets of ball bearings are provided, each supported by a pair of inner and outer races secured respectively to the shaft and housing. Solid spacer rings separate the races from one another and fix the races against axial movement. A mechanical seal is also provided to prevent the drilling fluid from entering the ball bearings. To transmit radial loads from the shaft to the housing, radial supports are provided at each end of the housing.
To assure proper loading of the thrust bearings, close tolerances are required. Since the components are axially fixed, loose tolerances would result in uneven loading and wear on the bearings. Close manufacturing tolerances, in turn, increase the cost of the drilling assembly. Accordingly, there is a present need to provide a bearing assembly which does not require expensive, exacting tolerances to evenly distribute loads to the bearings.
Another problem, with prior drilling assemblies such as the type referred to above, is that the drilling assembly is relatively long. Long drilling assemblies increase the radial loads on the shaft during deviation which, in turn, requires more or larger radial bearings in addition to the thrust bearings. Accordingly, there is a need for a bearing assembly which accommodates both thrust and radial loads to shorten the overall length of the drilling assembly and to reduce the radial bending loads.
Yet another drawback of the type of drilling assembly set forth above is that the thrust ball bearings engage races having flat faces. This results in wear being concentrated in essentially a line around the races. Concentrated wear shortens the useful life of the bearings. There is, therefore, a need for thrust ball bearings which distribute wear to lengthen the life of its bearings.
Still another drawback of drilling assemblies of the type described above is that when the thrust bearing races are worn, they must be discarded and replaced. Frequent replacement requires a stock of replacement races to be maintained at the site. This results in a need for races which, when worn, can be reused rather than discarded.